1. Field of the Invention
The invention relates to methods of producing circuit structures and somewhat more particularly to method of producing very fine circuit structures in microelectronics with very thin thin masks comprised of photosensitive resists or electron-or x-ray-sensitive resists.
2. Prior Art
Generally, in the production of fine and extremely fine structures in microelectronics, photo sensitive resists or resists which are electron or x-ray sensitive are utilized as masking. With increasing integration, the resist thickness, must also be reduced. Simultaneously, wet etching techniques are being more and more frequently replaced by dry etching techniques, such as plasma, sputtering or ion-beam etching. The last two dry-etching techniques operate with a directed ion beam which can bring about significant resist erosion, depending upon the energy level and ion type (argon, reactive gases, etc.) utilized. In addition, the erosion rate of a resist is strongly depended upon the angle of the incident ions and exhibits the largest erosion rate at approximately 60.degree. angles of incidents (deviations from the normals). This effect causes sloping resist flanks (60.degree.), which become more and more sloping with increased etching periods. The result of this is slanting edges on the etched layer and a lateral shrinkage of desired structure dimensions. The above problem is particularly noticeable when the material to be etched has no selectivity relative to the resist utilized (especially in instances of etching metals with argon) or when the layer to be etched is relatively thick.
A known solution to this problem is, as described for example by M. Cantagrel in IEEE Transactions on Electron Devices, Vol. ED-22 No. 7 (July 1975) pages 43-46, is to first apply a thin metal film (for example composed of Ti, V, Mn or Al) over the layer to be etched and structuring such film with a resist. This structured metal film then can function as a masking for the underlying layer. In this process, advantage is taken of the fact that for many metals, the etching rate of the metal oxide with Ar-ions is significantly less than that of the metal itself. In this manner, one can first, in a wet-chemical manner, by means of a plasma, or a reactive sputter-etching (i.e., RIE) by ion-beam or sputter etching with pure argon or by a "lift off" technique structure the corresponding metal film and then utilize it as a mask whereby further etching occurs with a mixture of Ar and about 10% O.sub.2. A metal oxide is then formed during this etching on the surface of the metal mask.
However, the foregoing method has several disadvantages. For example, the metal layer which functions as masking is very difficult to structure. Often one finds a thin oxide layer present, which makes the wet chemical or plasma etching (structuring) of the metal more difficult. Further, such oxide layer causes varying etching action which leads to undefined under-etching. On the other hand, in sputtering or ion-beam of such metal masks, the etchant selectivity relative to the resist utilized (particularly PMMA, ie., polymethylmethacrylate) is generally too low to prevent a resist shrinkage and thus a lateral shrinkage of the mask. A further disadvantage is that in the subsequent etching of the layer with an Ar-O.sub.2 mixture, only such materials can be etched which themselves do not form oxides which are resistant to Ar bombardment, for example gold.